The present invention relates to a liquid crystal display device and more particularly to a liquid crystal display device of the reflective type having an improved level of brightness.
The conventional reflective type TN mode and reflective type STN mode have been widely employed in portable type personal computers and word processors principally because they do not require backlighting and are of low power consumption. An example of a conventional liquid crystal display device 1 is illustrated in FIG. 27 with which the reflective type TN mode and the reflective type STN mode are employed. Liquid crystal device 1 includes upper and lower polarizing plates 2 and 3, reflecting plate 4 and a liquid crystal cell 1A. Liquid crystal cell 1A comprises upper and lower transparent substrates 11 and 12 spatially separated by means of spacer 20 forming an interior region for optically anisotropic liquid crystal material 15. A first transparent electrode 13 is disposed on the interior surface of upper substrate 11 and a second transparent electrode 16 is disposed on the interior surface of lower substrate 12. Orientation film 18 is disposed on the interior surface of electrode 13 and orientation film 19 is disposed on the interior surface of electrode 16. Reflecting plate 4 is provided relative to the exterior or outer surface of polarizing plate 3. Polarizing plates 2 and 3 are respectively disposed on the exterior surfaces of upper and lower substrates 11 and 12. This type of conventional double polarizer liquid crystal display device suffers from being too dark in displayed image and, especially in the case of the monochromatic black and white, reflective type STN mode, the displayed image is cause to be viewed in different colors on the display. Furthermore, there is the phenomenon of a double vision or imaging present in the displayed image, which is peculiar to the reflective type mode liquid crystal display device. The double image is caused by the presence of the lower display substrate which causes a reflected shadow of the displayed image from the lower substrate surface where the reflector is located. As a result, from the viewpoint of visual observation, there is a displayed image from the liquid crystal cell together with an offset shadow of the same image from the lower substrate, the offset being according to the thickness of the lower substrate.
FIG. 28 shows the spectral characteristics of the reflective type STN liquid crystal display device 1 in the case where an applied electric field to the display is turned ON and OFF. The display in this case is considered to be turned OFF when there is no or very little voltage applied to electrodes 13 and 16 resulting in no or insignificant change in orientation of the liquid crystal molecules within liquid crystal material 15. The display is considered to be turned ON when the applied voltage to electrodes 13 and 16 is sufficient to cause a change in orientation of the liquid crystal molecules within liquid crystal material 15. As a result, the optical transmission path of light through cell 1A is altered between conditions when the display is ON and OFF. In FIG. 28, the spectral characteristics indicated at 41 are the case when the electric field is turned ON and the spectral characteristics at 42 are the case when the electric field is turned OFF. An example of the conditions for this particular liquid crystal display device 1 is a twist angle of 255.degree., a retardation, .DELTA.nd, (wherein retardation, .DELTA.n.multidot.d, is the product of birefringence, .DELTA.n, and the layer thickness, d, of liquid crystal material 15) is 0.85 .mu.m, an angle formed relative to the polarizing or absorbing axis direction and the rubbing direction of 45.degree.. The colors present on the displayed image relative to the STN mode are yellow-green when the display is turned OFF and blue when the display is turned ON. Further, the average light reflectance from the display was only 65%, which is relatively low for good display visibility and contrast.
It is an object of this invention to provide a reflective type liquid crystal display device with substantially enhanced brightness of the displayed image.
It is another object of this invention to provide a reflective type liquid crystal display device, particularly of the TN and STN mode, that reduces, and can significantly resolve, multi-coloring effect, i.e., is substantially a monochromatic display of contrasting dark and light.
It is another object of this invention to provide a reflective type liquid crystal display device, particularly of the TN and STN mode, that reduces, and can significantly resolve, double vision or imaging of the displayed image.